The present invention relates to the distillation of cyclohexanol from phenol and especially to the separation of mixtures of cyclohexanol, cyclohexanone and phenol into an overheads fraction comprising cyclohexanol and cyclohexanone and a bottoms fraction containing phenol, cyclohexanone and a minor proportion of cyclohexanol. Such mixtures are found in the product of the hydrogenation of phenol to produce cyclohexanone, and especially in the bottoms from a product distillation which removes the major portion of the cyclohexanone from such hydrogenation product. The bottoms from the product distillation contains a minor proportion of cyclohexanone, the unreacted phenol and the cyclohexanol which has resulted from the overhydrogenation of phenol.
Cyclohexanone is produced commercially in large quantities for manufacturing caprolactam and other purposes by a variety of processes, with the reduction of phenol and the oxidation of cyclohexane being the most common. In the hydrogenation of phenol, the cyclohexanone must be recovered from a mixture which generally includes unreacted phenol and cyclohexanol resulting from the overhydrogenation of phenol. The mixture may also contain various impurities less volatile than phenol (hereinafter called high boilers) and various solvents such as cyclohexane, benzene or hexane which are more volatile than phenol.
Since cyclohexanone is the lowest boiling major component of the mixture, it is normal to first distill the major portion of the cyclohexanone from the reaction mixture leaving a bottoms fraction comprising phenol and cyclohexanol, and usually also cyclohexanone and a minor proportion of high boilers. Conventionally, this bottoms fraction is fed to a second fractional distillation column, either in batch operation or continuously, which is designed with sufficient numbers of distillation trays or equivalent contact devices such as packing and a sufficient reflux ratio to produce an overheads fraction containing the net cyclohexanol produced, some cyclohexanone and no more than trace quantities of phenol such as below 10 ppm, preferably below 1 ppm. The bottoms stream from this second column conventionally contains substantially all of the phenol and high boilers, some cyclohexanone and substantial amounts of cyclohexanol.
In the conventional second distillation, the bottoms stream cannot be reduced in cyclohexanol content below that of the maximum boiling azeotropic mixture of cyclohexanol and phenol. In actual operations, the cyclohexanol content of the bottoms stream significantly exceeds that of the azeotropic mixture. Since this second bottoms stream is normally recycled throughout an overall cyclohexanone recovery system (e.g. by subjecting this bottoms stream to further distillation, such as extractive distillation, and returning the overhead stream to a first, product column), an incentive exists for reducing the amount of cyclohexanol in the second bottoms stream, and thereby reducing the load and energy requirements of the overall system.
The operation of the second column is normally designed to assure that the phenol content of the second overheads is minimized. While it is desirable to also minimize the cyclohexanol content of the second bottoms stream, normal manipulation of distillation conditions has limited effectiveness in reducing the cyclohexanol content of the bottoms. This is due to the attractive forces between the phenol and cyclohexanol molecules that can result in the formation of a maximum boiling azeotrope under the conditions of the second distillation. In general, the feed to the second column, which is the bottoms from the first column, is to a point intermediate between the top and bottom of the second column, and generally somewhat below the midpoint of the second column. Thus, for example, in a thirty tray fractional distillation column, the feed can be to the tenth tray from the bottom. Because cyclohexanone is the most volatile of the three major components of the mixture, it generally is found in increasing quantities going up the column and decreasing quantities going down the column. Conversely, phenol, being the least volatile of the three components is conventionally found in increasing quantities going down the column and decreasing quantities going up the column. In order to minimize phenol content in the overheads, the column is run under conditions that cause the phenol content to reach negligible amounts at the top tray. Cyclohexanol, being intermediate in volatility, is distributed along the column in a manner satisfying the overall material balance. While the exact overall distribution of the cyclohexanol in the column has not been well understood, it has been known that a significant concentration of cyclohexanol is found in the bottoms stream.
Phenol mixes with cyclohexanol and with cyclohexanone in such a manner that the intermolecular forces cause the total vapor pressure of the mixture to be lower (at equilibrium) than the level that would be present if the mixture behaved ideally. This phenomenon, referred to as a negative deviation from ideal mixing or a negative deviation from Raoult's Law, is significant enough to result in a first azeotrope (a maximum boiling binary azeotrope) between phenol and cyclohexanone (at least throughout the common pressure range for this distillation) and to result in a second azeotrope (a maximum boiling binary azeotrope) between phenol and cyclohexanol.
Several processes have been proposed to recover phenol from cyclohexanone and/or cyclohexanol by extractive distillation: adding an extraneous component to break one or both azeotropes. Such processes have the disadvantage that the extraneous component must subsequently be separated from the desired components (i.e. cyclohexanone, phenol and/or cyclohexanone) or vice versa before the desired components can be used. Such processes also generally involve adding a less volatile component to the top of a column.